Transcript for Moment Deaf Toddler Hears for First Time With Brainstem Device

Healed and healthy Alex returns to the Massachusetts Eye and Ear Infirmary to half his -- BI switched on for the very first time. Those wires connect the device inside his head to a sound generated. Controlled from -- doctors computers in a moment they will switch it on with everyone hoping and watching to see if he reacts. But first the door opens to distill and Stephanie have decided they want the first thing Alex to -- to be his sister's voice yes. Yeah okay. It did not quite work out that way the device is on and the girls are talking but there's no reaction. And others in the room trying to reach in the -- coast watched with AT&T'S blocks. Against -- action. And then it happens to every -- surprised that doctor on the left nearly offscreen she is about to slam her -- to the side of her desk. And their -- that little turn of his head. And then he always son he do you run look. He got everything down from entering -- and -- doesn't -- he's definitely heard that like Kansas and I knew he was completely focused on the finger of on this sawing and then he just do you want.

This transcript has been automatically generated and may not be 100% accurate.

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